Transfer of energy between trophic levels is relatively inefficient. Energy is transferred from one trophic level to another as organisms are consumed.

In primary producers the main energy input is from the solar energy. In a plant, not all of the solar energy available actually makes it into the leaf.

There is loss of energy by reflection from the leaf, transmission through the leaf, and because some of the energy is the incorrect wavelength.

The energy that is taken up by the producer is then fixed by photosynthesis, although again a proportion of this energy is lost as it is used up during photosynthetic reactions.

Of the energy that is fixed in photosynthesis some will be used during respiration whilst the remaining energy is the portion that is incorporated into the biomass. It is the energy that is incorporated into the biomass that is available for the next trophic level.

In the consumer a further series of energy losses occur. The consumer will take in a certain amount of energy from the trophic level beneath it.

This energy intake does not equal the amount of energy available in the biomass of this organism since feeding is an inefficient process. There will be a loss of energy through the production of urine and faeces, as well as losses through respiration and heat loss. This leaves a proportion of the energy consumed to be incorporated into the biomass.

It is generally accepted that only around 10% of the energy gained from the previous trophic level is passed on to the next level. All other energy is lost as described above. This limits the number of trophic levels in any food chain.

Ecological pyramids are used as a tool to illustrate the feeding relationships of the organisms, which together make up a community.

Pyramid of numbers

This is the simplest way of illustrating the feeding relationships within a community. The commonest form shows that the numbers of organisms occupying each trophic level decreases from producers to secondary consumers and beyond.

Two problems with this form of pyramid are that the numbers involved may be huge (in the hundreds of thousands) and some pyramids may be inverted.

Pyramid of biomass

This indicates the feeding relationship between organisms occupying different trophic levels with reference to their biomass.

Biomass can be measured as either wet mass or dry mass. Measuring the dry mass is more accurate as it does not include the variable water content of organisms.

The commonest form of the pyramid of biomass shows that the total biomass of organisms occupying each trophic level decreases from producers to secondary consumers and beyond.

There is still the problem that a pyramid of biomass can be inverted and also it does not take account of changes over time. The sampling must all be carried out at one moment in time and therefore indicates the standing crop and not the productivity.

Pyramid of energy

This is the most accurate representation of the feeding relationship between the organisms at different trophic levels. It takes into account the energy gains and losses over a period of time.